This invention provides a method of cold extruding maraging steel by impact extrusion to form an article of manufacture. As used herein, the term "maraging steel" refers to a family of low-carbon, high-nickel, iron base alloys which typically contain about 18% nickel. Due to the high yield strength which can be developed in these materials, 18% nickel maraging steels have been specified for use in making sub projectile blanks for antitank cannon rounds. The sub projectile blank forms a critical part of the cannon round assembly because it is the carrier for a penetrator. The sub projectile blank has a cylindrical shape and is approximately thirteen inches long. The blank also includes a longitudinally extending, tapered hole which is approximately eleven inches long and, hence, extends almost the entire length of the blank. The tapered hole must be precisely located and have extremely close tolerances. The precise dimensions of the tapered hole are necessary for subsequent assembly procedures.
Heretofore, the blank has been machined from bar stock. Due to the toughness of maraging steel, machining is relatively difficult and time-consuming. Additionally, it is very difficult to maintain the close tolerances in the tapered hole which are required. Machining tends to produce surface stresses which promote stress-corrosion cracking in the final assembly. Machining also produces scrap in the form of chips. In machining the sub projectile blank approximately three pounds of scrap is produced. Since the current price for this material is approximately four dollars per pound, the elimination of scrap is economically significant.
In order to eliminate many of these problems and to reduce the cost of producing sub projectile blanks, efforts have been made to employ impact extrusion techniques to form the blanks. Up until now, however, attempts to cold extrude 18% nickel maraging steel in this fashion have been unsuccessful. All previous known attempts to impact extrude sub projectile blanks from 18% nickel maraging steel have resulted in tool breakage.